The present invention relates to an apparatus for introducing gas, for example air, oxygen, methane, etc., into a liquid mass, at a certain or particular depth therein, thereby making it possible to diffuse the gas into the liquid, while simultaneously obtaining a desired effect of mixing and homogenization. This apparatus is most particularly usable in sewage treatment installations involving activated sludge. However, it is intended that the scope of the present invention be applicable to any type of installation requiring a gas diffusion in a liquid, at a certain depth therein.
There are known devices of the type which are immersed in a liquid, particularly devices for the diffusion of air in water treatment tanks employing activated sludge. In this type of device, air is pumped from the atmosphere by a compressor through a piping system which releases the air in a liquid in rather fine bubbles. The use of this type of apparatus necessitates a compromise between a high transfer yield, i.e. the transfer to the liquid in dissolved form of the maximum percentage of gas blown in, with a mixing capacity sufficient to obtain adequate homogenization of the liquid mass and perhaps also to suspend solid matter within the liquid mass, a very important feature in the case of activated sludge systems. Moreover, the consumption of energy must be maintained at a minimum.
The simplest known method for introducing gas into a liquid involves the use of a tubing or piping system which is immersed horizontally within the liquid and is fed with the gas, for example air, and frees the gas through calibrated orifices which are regularly spaced along the line of the piping system. The gas bubbles released, as they rise toward the surface of the liquid, surrender a portion of their constituent elements to the resultant rising stream, yielding oxygen for example, and give rise to circulation of the liquid mass by virtue of the ascending effect of low-density emulsified zones. This type of device has the disadvantage of having a low energy transfer yield and of resulting in inadequate mixing, particularly in the bottom areas of the tank, whenever the demand for gas is slight and requires only a reduced gas flow.
Another known device makes it possible to improve mixing without increasing the flow of gas and includes a vertical tube, open at both ends, designed to channel the injected gas, thereby serving as an emulsion pump and ensuring a substantial transit flow. This type of system makes it easier to ensure the taking up and recirculation of the lower levels of the liquid mass than with the known device described above, with equal flows of air or other gas. However, the transfer yield of this known device is no better, and in fact the improvement in gas transfer, oxygen transfer for example, stemming from the high level of turbulence in the tubing is cancelled out by the reduced period during which the gas bubbles remain in the liquid mass, since the gas bubbles rise more rapidly to the open upper surface of the liquid.
A third type of known device includes an emulsion tube containing in the interior thereof baffles of various shapes designed to promote turbulence and to create a load loss in order to increase the time of stay of the bubbles within the liquid gas. The transfer of gas, such as oxygen, is thus improved. However, this type of device has the drawback of slowing the flow of gas from the tube, and thus the pumping and mixing capabilities of this type of device are limited.